Although fossil fuels are currently the world's major source of energy, the increasing cost of fossil fuels and the increasing uncertainty regarding the availability of oil, the most readily used form of fossil fuels, have generated an ever increasing demand for alternative forms of energy. Concern over the environmental impact of coal and nuclear energy, as well as concern over the safety of nuclear energy has led to a growing interest in solar energy.
The approaches to harnessing solar energy have been many and varied. Of most pertinence to the present invention are thermal approaches utilizing solar heat collectors and thermodynamic motors or heat engines.
Typically, conventional solar heat collectors basically comprise a heat transfer fluid, commonly water, and some form of solar absorber apparatus for converting solar radiation to heat radiation, although in some solar heat collectors the heat transfer fluid also functions to convert the solar radiation to heat radiation. Examples of the former type of collector are disclosed in U.S. Pat. Nos. 3,215,134 (Thomason); 3,965,887 (Gramer et al.); 4,030,478 (Beaver, Jr.); 4,038,967 (Stout et al.); 4,067,316 (Brin et al.); 4,086,912 (Freeman); and 4,106,482 (Savage et al.). Examples of the latter type of collector are disclosed in U.S. Pat. Nos. 4,062,351 (Hastwell); 4,134,389 (McClintock); and 4,158,355 (Spitzer). Of the collectors which segregate the heat collecting and heat transfer functions, several shield the heat transfer medium from exposure to the sun. The collectors disclosed in the Gramer et al., Stout et al., Brin et al., and Savage et al. patents are of this type. The efficiency of such collectors is limited because there is nothing to usefully absorb the heat which is radiated from the collector back toward the sun and away from the heat transfer medium.
Several specific features have been incorporated into conventional solar heat collectors in order to improve the efficiency thereof. As exemplified by the Thomason collector, the heat collector surface has been configured in an irregular, nonplanar shape to provide for channelization of the heat transfer liquid so as to minimize the volume-to-surface ratio thereof. An irregular configuration has also been employed wherein a plurality of angularly disposed planar surfaces are provided which allow the solar angles of incidence either to be decreased or to be increased to cause any reflection of solar radiation from the collector surface which occurs to be back onto the collector surface rather than back to the atmosphere. However, the efficiency of such collectors is reduced because a uniform fluid flow over the entire heat absorbing surface has heretofore not been obtained.
A further feature of conventional solar collectors is the provision of a relatively transparent sheet, which may be in the form of a flexible membrane, overlying or covering the absorber surface. One common function of such a sheet is to reduce uncontrolled reradiation and convection currents. Another common function is to decrease the undesired effects of vaporization of the heat transfer fluid. However, conventional collectors having such covering sheets lose efficiency through absorption of solar energy in those sheets which are thick enough to withstand the pressures created within the collectors when the heat transfer fluid is pumped therethrough at an accelerated rate, or through the inability of those sheets which are flexible to contain the heat transfer fluid properly when a flow rate greater than a trickle is desired or when those collectors utilizing a minimal flow rate are tilted at steep angles. Another disadvantage heretofore of using covering sheets with collectors having channelized fluid flow has been the necessity of mechanically sealing the covering sheets to irregularly configured collector surfaces in order to adequately contain and control the flow.
Another approach to improving the efficiency of conventional solar collectors is disclosed in the Gramer et al. patent. Thermal transfer is improved between the solar absorber and the heat transfer liquid by providing rectangular fluid passages having specified dimensions, and by accelerating the heat transfer fluid flow through the passages to maintain a laminar flow of substantially constant cross-sectional area, so as to minimize eddy currents.
Yet another approach to improving the efficiency of conventional solar collectors has been to provide tracking apparatus for maintaining the collector in a predetermined orientation with respect to the sun during the transit thereof through the sky. Examples of such trackers are disclosed in U.S. Pat. Nos. 1,938,003 (Arthuys et al.); 4,044,752 (Barak); and 4,147,154 (Lewandowski).
However, despite these and other features, conventional solar heat collectors have not achieved sufficiently high operating efficiencies to justify the costs of manufacture, installation, and operation for widespread use.
With regard to conventional heat engines, such devices have employed either a single volatile fluid or a volatile fluid which is mechanically coupled to a dense fluid. Examples of single-fluid heat engines are disclosed in U.S. Pat. Nos. 243,909 (Iske et al.); and 3,983,704 (McFarland); and in an article entitled "Wally Minto's Wonder Wheel," appearing in the March 1976 issue of Popular Science at page 79. Examples of two-fluid heat engines are disclosed in U.S. Pat. Nos. 3,984,985 (Lapeyre); and 4,074,534 (Morgan). However, both types of heat engines have exceedingly low operating efficiencies and have never proved commercially feasible.